1. Field of the Invention
This invention relates to apparatus for positioning and releasing a prosthetic acetabular cup within an acetabulum during total hip replacement surgery.
2. Description of the Prior Art
As part of total hip replacement surgery, a prosthetic acetabular cup is implanted within the human acetabulum to substitute for the socket of the human hip joint. This is usually done to compensate for severe damage of the acetabulum due to disease, trauma, or other factors. A prosthetic femoral component is mated with the acetabular cup component to complete the total hip replacement surgery. In order to achieve optimal performance of the combined acetabular and femoral prostheses, the acetabular cup must be properly positioned in the acetabulum. It is also important that a properly positioned acetabular cup not be loosened or dislodged while insertion instruments are being removed. An improperly positioned or a loosened acetabular component can lead to dislocations of the hip joint, decreased range of motion, and eventual failure of both the acetabular and femoral components.
Interoperative placement of the acetabular component can be a surgically demanding task. The orthopedic surgeon must adequately fix the cup in the proper alignment while hampered by limited surgical exposure and relatively few clear reference points. A wide variety of insertion instruments are known in the art for inserting acetabular cup implants into position in the acetabulum. Many such insertion instruments include a first handle with a second handle extending therefrom to assist in the insertion procedure. Typically, the second handle is in a fixed orientation relative to the first handle. Examples of such acetabular instruments are disclosed in U.S. Pat. Nos. 4,528,980 to Kenna; 4,475,549 to Oh; 4,305,394 to Bertuch, Jr.; 3,859,992 to Amstutz; and 4,632,111 to Roche.
Acetabular cups known in the art are generally provided in the form of an outer shell preassembled to a polyethylene bearing insert. Alternately, the cup shell and the bearing insert are provided as separate pieces to be assembled after the shell is affixed via bone screws or other means.
Once the acetabular cup is attached to an insertion instrument, the handles are in a fixed orientation with regard to the cup. With certain cup designs a particular orientation of the cup may be desirable to achieve proper anatomic positioning. An example of such a cup design is a cup which has pegs protruding from the outer surface, such as the pegged cup disclosed in U.S. Pat. No. 3,781,918 to Mathys. When the first and second handles of an insertion instrument are both in a permanently fixed orientation with regard to the cup, the second handle may be in an awkward or inconvenient position for inserting the cup in the desired orientation. Also, if the first handle needs to be rotated about its axis during the insertion of the cup or during the removal of the insertion instrument from the cup, a fixed secondary handle may get in the way.
Traditional cup positioners generally include at least a pusher ball which is sized and shaped to fit the recess within the acetabular component, a positioner arm, a positioner flange juxtaposed the pusher ball, and a pusher arm connected to the ball and to the flange to enable the user to push the prosthetic acetabular cup, when resting on the flange, into the prepared acetabulum. Generally, the cup must be manually held against the flange during implantation. In some cases, the cup is aligned on the flange by means of small projecting pins which can engage complementary holes in the face of the cup.
Separate cup pushers are often used in conjunction with a cup positioner to address some of these problems. The positioner is removed once the cup has been placed in the acetabulum, and the pusher is separately used to apply pressure to the cup.
In addition to the pusher ball, the flange, and the pusher arm, some acetabular cup positioning instruments include means for establishing reference angles to better ensure proper alignment of the prosthetic cup. Many traditional positioner designs include a reference arm for establishing the abduction angle. U.S. Pat. No. 4,475,549 to Oh further includes means for establishing the anteversion angle. (All of the documents identified herein, including the foregoing, are incorporated herein by reference in their entireties for all purposes.)